Human blood platelets are important in the functioning of the normal hemostatic mechanism. In addition, they are suspected of playing a central role in the development of arteriosclerosis and other cardiovascular diseases. As they perform their functions, platelets undergo dramatic changes in shape. When exposed to damaged blood vessels, for example, long pseudopods extend from the cell within seconds. At the same time, the platelets adhere to each other and to the vessel wall. These pseudopods can then be retracted, and appear to be responsible for the platelet's ability to generate contractile force. Polymerization and depolymerization of actin filaments is thought to play an important role in these shape changes and force generation. The long term goal of this project is to develop an understanding of how these cycles of polymerization and depolymerization are controlled. We have recently studied a protein preparation from muscle which has the ability to interact specifically with the morphologically defined "pointed" end of the actin filament in a way that inhibits polymerization at that end. Other investigators have described a protein called acumentin from leukocytes which appears to have similar activity. The immediate goal of this study will be to determine the role these proteins play in the control of actin polymerization. This will be accomplished by further purifying the pointed end inhibitor and examining platelet extracts for proteins which are cross-reactive with the muscle and leukocyte proteins. Attempts will also be made to identify pointed end inhibitor activity in platelets by the same protein purification procedures that were successful in identifying the pointed end inhibitor from muscle. The platelet proteins would then be compared in detail to the muscle and leukocyte inhibitors by a variety of biochemical and functional parameters. The molecular mechanisms of the pointed end capping proteins will also be explored using biophysical techniques such as light acattering, fluorescent energy transfer, and scanning transmission electron microscopy. The possibility that these proteins are regulated by factors such as pH, calcium, and phosphorylation will also be investigated. Studies of highly selected patients with possible abnormalities of platelet structure and contractile function will also be considered. These studies should contribute to our understanding of the physiology of human blood platelets, and provide insight into principles applicable to the field of muscle and nonmuscle motility in general.